1. Field of Invention
This invention relates to a Yagi-Uda (Yagi) antenna, and in particular, to a Yagi antenna formed on a printed circuit board having matching coaxial cable and driven element impedances.
2. Discussion of Related Art
The traditional Yagi antenna encompasses a broad class of antennas which usually have one active dipole element (sometimes referred to as the "driven element"), one reflector dipole element, and one or more director dipole elements. A typical arrangement is shown in FIG. 7. The traditional Yagi antenna is constructed with a longitudinal support structure 102 having dipole elements 103-105 arrayed in a fishbone pattern. The support structure 102 can be made from any rigid material including metal. Director elements 105 and reflector element 104 may be attached directly (electrically) to the metallic support structure 102 without affecting the antenna performance since the mid-points of these elements are at a negligible potential. The active dipole element 103, however, is isolated from the metallic support structure. The directional and bandwidth characteristics are determined primarily by the element spacings and lengths. A common arrangement is for the reflector length Lr to be slightly larger than 1/2.lambda., and for the director lengths L1-L3 to be slightly less than 1/2.lambda..
As shown in FIG. 7, the driven element 103 of a conventional Yagi antenna is typically a simple half-wave dipole or a folded dipole and is driven by a source of electromagnetic energy. The plurality of director elements 105 are disposed on one side of the driven element 103 while the reflector element 104 is disposed on the other side of the driven element 103. The director elements 105 are usually disposed in a spaced relationship in the portion of the antenna pointing in the direction to which electromagnetic energy (radio waves) is to be transmitted, or the direction from which radio waves are to be received. The reflector element 104 is disposed on the side of the driven element 103 opposite from the array of director elements 105.
When the driven element 103 radiates, it induces electrical currents to flow in the parasitic elements 104 and 103 which in turn cause the parasitic elements to re-radiate. If the antenna is being used to transmit, the director elements 105 are positioned so that the radio waves re-radiating from the director elements 105 constructively combine with the radio waves radiating from the driven element 103, thereby focusing the combined radio waves in a specific direction. Thus, the operation of the directors is analogous to the operation of an optical lens.
Conversely, the reflector element 104 is positioned so that it re-radiates radio waves 180.degree. out of phase with the radio waves generated by the driven element 3, thereby creating an electrical null. Thus, the operation of the reflector 104 is analogous to the operation of a mirror. If the antenna is being used to receive radio waves, the director elements 105 focus signals received from a specific direction to the driven element 103 while the reflector element 104 cancels radio waves received from the opposite direction.
The Yagi antenna has been used successfully in applications such as reception of television signals, HAM radio, point-to-point communications, and other applications requiring high directivity or gain in a particular direction. This directivity offers the advantage of increased antenna gain in one direction and decreased antenna gain in other directions. Therefore, weak signals may be received at a higher signal strength by pointing the antenna towards the signal source. Similarly, when used to transmit signals, the Yagi antenna provides increased effective transmit power in a given direction.
An antenna has an input impedance which is usually measured looking into the driven element. Each type of driven element has a particular free space impedance. For example, the impedance of the conventional Yagi antenna at a folded dipole driven element is typically 300 .OMEGA.. A standard coaxial cable, used to connect the antenna to a receiver or transmitter, has either a 50 .OMEGA. or 72 .OMEGA. impedance. If, for example, a 50 .OMEGA. cable is connected to a 300 .OMEGA. antenna, the impedance mismatch causes a large percentage of the electromagnetic energy to be reflected back toward the energy source thereby decreasing the antenna performance and gain. Therefore, it is desirable to match the antenna impedance to the impedance of the coaxial cable.
One conventional solution to this impedance matching problem is to provide a matching network between the driven element and the antenna cable. Powers et al., U.S. Pat. No. 5,061,944 discloses such a matching network. Furthermore, Powers et al. discloses a particular type of driven element known as a balanced feed. The use of a balanced feed adds the additional requirement that a balance-unbalanced transformer (balun) be inserted between the coaxial cable and the driven element. The requirement of adding an impedance matching network and a balun to the antenna increases component count, cost, and assembly time, and may limit the frequency response of the antenna.
The spacings between the elements of conventional Yagi antennas are dictated primarily by the wavelength .lambda. of the transmitted or received radio waves because the parasitic elements are designed to be a sufficient distance, relative to .lambda., from the driven element (e.g., a folded dipole) and from other adjacent parasitic elements. Thus, for a given number of directors, the size reduction of the antenna is limited. One approach to overcoming the minimum element spacing required by traditional Yagi antennas is to use a 72 .OMEGA. simple dipole and to reduce the spacing between adjacent parasitic elements and between parasitic elements and the driven element. An additional advantage of this approach is that the impedance of the driven element can be reduced to match the impedance of the antenna cable, thus eliminating the need for a matching network.
This impedance reduction is due to the advantageous loading effects on the driven element by the closely coupled parasitic elements. However, the use of the 72 .OMEGA. simple dipole does not allow for tightly coupled element spacing and suffers from poor directional gain. While the use of a 300 .OMEGA. folded dipole offers better directional gain, this design suffers from extreme sensitivity to small variations in element spacing. Thus, neither the simple dipole nor the folded dipole are optimally used as the driven element to provide good directional gain with tightly coupled elements.
One solution is to use a partial folded J element, having an impedance of 150 .OMEGA., as the driven element as disclosed in the publication "Antennas, Selection, Installation and Projects," Evans and Britain, 1998. The use of the partial folded J element reduces antenna performance sensitivity to small changes in element spacing. The J element provides excellent directional gain due to tight element coupling. More importantly, the J element can be loaded by reducing parasitic element spacing so that its input impedance is substantially equal to the impedance of the coaxial feed cable. Consequently, the feed cable is attached directly to the driven element. However, this design is assembled by hand and is not reliably and repeatably manufactured at low cost, particularly at short wavelengths.
As the tuned frequency of the antenna is increased, the wavelength .lambda. decreases thus requiring smaller and smaller element dimensions and spacing. Because at high frequencies small variations in the length and spacing of the antenna elements cause changes in electromagnetic characteristics, discretely assembled antennas can have significant variations in performance. For example, the conventional Yagi antenna, shown in FIG. 7, has a metallic support structure and the elements are attached by hand via screws, rivets or other attaching means and methods. The variation in the antenna due to hand assembly leads to increased production cost and a low measure of repeatability.
One solution to this inefficient assembly problem is to form the antenna on a printed circuit board, as disclosed in Skladany, U.S. Pat. No. 5,712,643 and Shafai, U.S. Pat. No. 5,896,108. However, the antenna disclosed in Shafai requires a matching network between the driven element and the signal source, thus adding to the component count and cost. The antenna disclosed in Skladany consists of two printed circuit boards with one of the printed circuit boards having a hybrid coupler feed serving as a balun. These extra components add significantly to production cost and assembly time. Furthermore, in the designs disclosed by both Skladany and Shafai, the minimum required spacing between adjacent parasitic elements and between parasitic elements limits a reduction in overall antenna size. For example, the balanced feed of the Skladany antenna does not allow for close coupling of the Yagi elements thereby resulting in an antenna design that is larger than necessary.
One major difficulty in designing a printed antenna is finding a simple and inexpensive method of attaching a coaxial feed cable directly to the antenna elements. One conventional approach requires that the coaxial cable be attached to both the top of the printed circuit board via the coaxial cable's center conductor, and the bottom of the printed circuit board via the coaxial cable's ground mesh. This approach complicates the assembly procedure because a soldering or attaching operation must be performed on both sides of the printed circuit board. Other methods of attaching the coaxial cable to the printed antenna require that the cable be attached to the antenna at a 90.degree. angle relative to the major surface of the printed circuit board, thereby limiting design choices to this particular topology.